Enable flake8 for MPCD #5215

Job	Run time
Start action runners	53s
Build [${{ join(matrix.config, '_') }}]	0s
Build [clang14_py311_mpi_tbb_llvm]	4m 44s
Build [gcc13_py312]	4m 16s
Build [gcc13_py312_nomd]	3m 11s
Build [gcc13_py312_nohpmc]	3m 11s
Build [gcc13_py312_nomd_nohpmc]	1m 27s
Build [cuda120_gcc11_py310_mpi_llvm_debug]	12m 31s
Build [cuda120_gcc11_py310_mpi_llvm]	11m 20s
Build [cuda120_gcc11_py310]	10m 10s
Build [gcc9_py39]	3m 50s
Run ctest [${{ join(matrix.config, '_') }}]	0s
Run pytest [${{ join(matrix.config, '_') }}]	0s
Run ctest [clang14_py311_mpi_tbb_llvm]	1m 49s
Run pytest [clang14_py311_mpi_tbb_llvm]	2m 0s
Run ctest [gcc13_py312]	1m 3s
Validate [${{ join(matrix.config, '_') }}]	0s
Run pytest [gcc13_py312]	1m 13s
Run ctest [gcc13_py312_nomd]	1m 1s
Run pytest [gcc13_py312_nomd]	1m 9s
Run ctest [gcc13_py312_nohpmc]	1m 5s
Run pytest [gcc13_py312_nohpmc]	1m 7s
Run ctest [gcc13_py312_nomd_nohpmc]	1m 8s
Run pytest [gcc13_py312_nomd_nohpmc]	1m 5s
Run pytest [cuda120_gcc11_py310_mpi_llvm_debug]	9m 30s
Run pytest [cuda120_gcc11_py310_mpi_llvm]	6m 14s
Run ctest [cuda120_gcc11_py310_mpi_llvm_debug]	5m 1s
Run pytest [cuda120_gcc11_py310]	2m 33s
Run ctest [cuda120_gcc11_py310_mpi_llvm]	2m 10s
Run pytest [gcc9_py39]	1m 13s
Run ctest [cuda120_gcc11_py310]	1m 19s
Run ctest [gcc9_py39]	57s
Unit test	1s
	1h 37m 11s

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